The present invention is directed generally to a Hall-effect element with offset control and a method for operating a Hall-effect element to reduce null offset, and, in particular, a Hall-effect element with integrated offset control and a method for operating a Hall-effect element having integrated offset control to reduce null offset.
Hall-effect elements are well known in the art. Hall-effect elements rely on a reaction between a current flowing between a first set of contacts and an orthogonally-applied magnetic field to generate a voltage across a second set of contacts.
In theory, with no magnetic field applied to the Hall-effect element, no voltage should be generated across the second set of contacts. In practice, a voltage is typically generated across the second set of contacts even with no magnetic field applied to the Hall-effect element. This voltage is referred to as null offset.
There are several reasons for the deviation between theory and practice. For example, process variations in the fabrication of the Hall-effect element may cause local variations in resistance. Stress across the element in either wafer or packaged form may also cause local variations in resistance because of piezoresistive effects. Further, unbalance in the subsequent amplifiers typically used with Hall-effect elements also can contribute to null offset.
While the null offset is usually quite small, it still can be large enough to affect the proper operation of the Hall-effect element. As a consequence, a trimmer network is typically provided to balance the resistance variations. For example, the trimmer network may include a current mirror with trimmable degeneration resistors.
Trimmer networks come with their own set of associated problems. For example, Hall-effect elements are generally fabricated using a lightly doped n-type layer for heightened sensitivity to variations in magnetic field intensity. This means, however, that the element is also particularly sensitive to variations in temperature. As a consequence, the trimmer network must be able to adapt to the variations in resistance accompanying the variations in temperature. Furthermore, the subsequent amplifiers may also have variations with temperature which require compensation.
One way to make the trimmer network responsive to variations in resistance accompanying the variations in temperature is to provide a large reference resistor in close thermal proximity to the device that requires temperature compensation. For example, the reference resistor may be disposed in close proximity to the Hall-effect element, and coupled to the associated trimmer network. In operation, the trimmer network directs a portion of the current flowing through the reference resistor through the one of the Hall-effect element contacts to compensate for temperature-dependent changes in resistance. If variations occur in subsequent amplifiers or comparators, reference resistors must be provided for these devices as well.
Another way to achieve temperature compensation involves using switches on the same chip as the Hall-effect element to direct current alternatively to the separate sets of Hall-effect element contacts. The output voltage difference between the two contacts is stored on a capacitor to allow subsequent compensations of the Hall-effect element.
Disadvantageously, these trimmer networks with their associated compensation devices increase the size and complexity of the Hall-effect elements, both in terms of fabrication and operation.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
According to an aspect of the present invention, a Hall-effect element includes an isolating layer and an active layer of a first electrical conductivity type disposed on the isolating layer, the active layer having a surface. A first set of contacts is disposed in contact with the surface along a first axis, and a second set of contacts is disposed in contact with the surface along a second axis transverse to the first axis. An insulating layer is disposed on the surface, and a metal control field plate is disposed on the insulating layer. A voltage source is selectively coupleable to the metal control field plate to control the accumulation of charge carriers at the surface of the active layer to vary the resistance of the active layer.
According to another aspect of the present invention, a Hall-effect apparatus includes a Hall-effect structure, a first set of contacts disposed on the structure along a first axis, and a second set of contacts disposed on the structure along a second axis transverse to the first axis. A metal control field plate is disposed in relation to the structure such that, when a voltage is selectively coupled to the metal control field plate, the Hall-effect apparatus has a zero output in the absence of a magnetic field.
According to a further aspect of the present invention, a method is provided for reducing null offset in a Hall-effect element. The method includes the steps of providing an isolating layer, disposing an active layer of a first electrical conductivity type on the isolating layer, the active layer having a surface, disposing a first set of contacts on the surface along a first axis, disposing a second set of contacts on the surface along a second axis transverse to the first axis, and disposing an insulating layer on the active layer. A voltage is applied across the insulating layer to control the accumulation of charge carriers at the surface to vary the resistance of the active layer.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.